1. Field of the Invention
The present invention relates to a hydraulic disk brake.
2. Description of the Related Art
An example of hydraulic disk brakes in which to be applied to braking devices of a vehicle such as automobiles, autobicycles, etc. will be explained. The hydraulic disk brake comprises: a disk rotor that rotates together with a wheel; a pair of brake pads arranged at each side of the disk rotor; a caliper provided with a cylinder bore that faces the rear side of the brake pad, the caliper being supported on a side of a vehicle body; and a piston that is slidably inserted into the cylinder bore so as to press the brake pads to the disk rotor. The piston moves forward with hydraulic pressure supplied from a hydraulic pressure source such as a master cylinder to the cylinder bore, whereby braking force will be generated by pressing the brake pads against the disk rotor.
In general, as shown, for example, in Japanese Patent Application Laid-Open No. 2006-105169 (hereinafter referred to as Patent Document 1), a piston seal that is rectangular in section and made of an elastic material such as EPDM (ethylene-propylene rubber) is adapted to seal between a cylinder bore and a piston of a hydraulic disk brake.
Conventional piston seals will be discussed with reference to FIGS. 8 to 13. As shown in FIG. 10, a piston seal 1 is fitted into a sealing groove 3, the sealing groove 3 being an inner periphery groove in a rectangular section and being formed on an inner periphery of a cylinder bore 2. Further, the piston seal 1 is attached to an external periphery of a piston 4 so as to seal between the cylinder bore 2 and the piston 4. In the sealing groove 3, its bottom surface portion 3A is formed so as to make a diameter of the sealing groove 3 expanded as moving toward an opening of the cylinder bore 2 (left side in FIG. 10) whereby the sealing groove 3 is formed approximately in a trapezoidal section. Moreover, dimension of the sealing groove 3 in an axial direction is formed to be larger than the one of the piston seal 1. When the piston seal 1 is fitted into the sealing groove 3 as shown in FIG. 9, the piston seal 1 will abut against a lateral surface portion 3B placed on an opening side of the cylinder bore 2 due to an inclination of the bottom surface portion 3A of the sealing groove 3. The piston seal 1 is thus deformed along the inclination of the bottom surface portion 3A whereby the inner peripheral surface thereof will be tapered. By forming the inner peripheral surface of the piston seal 1 into the tapered configuration, it is possible to enhance adhesive properties between the piston seal 1 and the piston 4 while being able to reduce sliding-resistance therebetween. At each marginal portion of the sealing groove 3, chamfered portions 5, 6 are formed.
When braking is applied, the piston 4 moves forward by hydraulic pressure supplied into the cylinder bore 2, whereby the piston seal 1 is elastically deformed in a left-sided direction along the chamfered portion 5. On the other hand, when the braking is released, the piston 4 is adapted to move backward in a right-sided direction due to a rollback function to which the elastic quality of the piston seal 1 is participated. Here, in case brake pads are worn down, the piston 4 is adapted to move forward while braking is applied only for an amount where the brake pads are worn down. In the above condition, the piston 4 is adapted to slide relative to the piston seal 1. This makes possible to compensate the brake pads' wear making a pad clearance adjustable to a constant amount.
In the above-described conventional disk brake provided with the piston seal, however, the following problems may occur. For example, as shown in FIG. 8, in case that an inclined angle of the bottom surface portion 3A of the sealing groove 3 is set to 10 degrees relative to a line parallel to the axis of the cylinder bore 2 while an angle defined between the bottom surface portion 3A and the lateral surface portion 3B is set to 80 degrees, the following can be stated. That is, as shown in FIG. 9, when the piston seal 1 is fitted into the sealing groove 3, a clearance C will exist between the bottom surface portion 3A and the piston seal 1, as well as between the lateral surface portion 3B and the piston seal 1. This clearance C remains, as shown in FIG. 10, even if the piston 4 is installed into the cylinder bore 2.
In the above condition, if hydraulic pressure is supplied into the cylinder bore 2, the piston seal 1 will be deformed up to an amount defined by the clearance C due to the hydraulic pressure supplied. Accordingly, when the piston seal 1 abuts against the bottom surface portion 3A and the lateral surface portion 3B of the sealing groove 3, hydraulic-pressure variation occurs causing deterioration in brake operationabilities. Especially, disk brakes that apply to autobicycles where no booster is provided while hydraulic pressure is directly generated through manual operations tend to cause deterioration in the brake operationabilities.
Still further, as shown in FIG. 11, for example, in case that an inclined angle of the bottom surface portion 3A of the sealing groove 3 is set to 5 degrees relative to a line parallel to the axis of the cylinder bore 2 while an angle defined between the bottom surface portion 3A and the lateral surface portion 3B is set to 90 degrees, the following can be stated. That is, as shown in FIG. 12, in case that the piston seal 1 is fitted into the sealing groove 3, the bottom surface portion 3A and the lateral surface portion 3B of the sealing groove 3 and the piston seal 1 that is rectangular in section will abut to each other. However, even in this case, as shown in FIG. 13, when the piston 4 is installed into the cylinder bore 2, the piston seal 1 is deformed so as to provide the clearance C between an opening side of the lateral surface portion 3B of the sealing groove 3 and the piston seal 1. Accordingly, also in this case, when hydraulic pressure is supplied into the cylinder bore 2, the piston seal 1 needs to be deformed up to an amount defined by the clearance C thereby causing problems as to deterioration in the brake operationabilities.